I have a new MS 251 and wanted to replace the sprocket, bar and chain. In looking for parts, I could not find any sites that had the 251 drum/rim option for the 251. Baileys, etc. had nothing listed. I even stopped into my local dealer and asked and after doing some looking he "guessed" that the 250 and 251 would be the same. I left without buying it. I did measure and they are very, very close, so close the difference is probably just the difference between Stihl and the Oregon replacement. I went ahead and bought an Oregon set up for the saw and it seems to run fine, I haven't cut with yet but hope to get out and do that today. If it blows up I'll check back in.